The present invention relates to a novel method of removing haze from plastic articles made of cellulose acetate butyrate, particilarly light polarizing sunglass lenses made of cellulose acetate butyrate and polyvinyl alcohol, which haze has been created during exposure of said plastic articles to humidity in the air or during dyeing processes.
In many plastic articles made of cellulose acetate butyrate, clarity or transparency of the article is very important and haze is undesirable or intolerable. This is particularly true in the case of light polarizing sunglass lenses comprising a central layer of oriented polyvinyl alcohol (PVA) and a substrate layer of cellulose acetate butyrate (CAB) on either side of the PVA layer. Light-polarizing sunglass lenses having said construction are very common and occupy a predominant share of the present market of the light-polarizing sunglass lenses (polarizing lenses, hereafter). Said type of polarizing lenses, however, has a disadvantage that after a long exposure to high humidity air, particularly at higher ambient temperatures, they tend to develop haze. This can take place during storage or transportation and is undoubtedly due to an interaction between water and the lens material. Haze development in the polarizing lenses can occur also during the dyeing process if a water based dye bath is used. Such haze development is obviously very detrimental to the value of the sunglass lens and if the haze is excessive, the lens must be discarded resulting in a substantial economic loss when the quantity is large. Of course, haze development is undesirable not only in the case of polarizing lenses, but also in the case of many other plastic articles made of CAB.
We have discovered that haze created in plastic articles made with CAB, including polarizing lenses, during exposure to humidity or during dyeing processes is not an irreversible impairment, but instead can substantially be removed by immersing such hazed articles in a substantially water-free bath composition at least 70 weight percent of which is a combination of an active solvent and a carrier solvent, the former being methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol or any mixture thereof, and the latter being ethylene glycol, glycerol, tricresyl phosphate, butyl phthalate, or any mixture thereof, and the relative proportions between the former and the latter being in the range of 90-5 and 10-95 weight percent, respectively, and more preferably 80-10 and 20-90 weight percent, respectively.
The applicability of the haze removal method of this invention is not limited to the polarizing sunglass lenses made of CAB, but rather the method is generally applicable to plastic articles made of CAB where haze has been created due to water.